| Abstract
| - Ab initio calculations predict that homolytic substitution reactions involving silyl, germyl, and stannyl radicals with disilane, digermane, distannane, silylgermane, silylstannane, and germylstannane can proceed by mechanisms which involve both backside and frontside attack at silicon, germanium, or tin.
- Ab initio calculations using the 6-311G**, cc-pVDZ, aug-cc-pVDZ, and (valence) double-ζpseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (UHF) theinclusion of electron correlation, predict that degenerate homolytic substitution by silyl radicalat the silicon atom in disilane can proceed by mechanisms which involve both backside andfrontside attack at silicon. At the highest level of theory (CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ), energy barriers (ΔE⧧) of 52.7 and 58.2 kJ mol-1 are calculated for the backsideand frontside reactions, respectively. Similar results are obtained at the CCSD(T)/DZP//MP2/DZP level of theory for reactions involving germanium and tin with values of ΔE⧧ of65.2 kJ mol-1 (backside) and 76.7 kJ mol-1 (frontside) for reactions of germyl radical withdigermane and 58.5 kJ mol-1 (backside) and 59.1 kJ mol-1 (frontside) for reactions of stannylradical with distannane. CCSD(T)/DZP//MP2/DZP calculations involving the analogousnondegenerate reactions of disilane, digermane, and distannane, as well as reactionsinvolving silylgermane, silylstannane, and germylstannane, predict that while homolyticsubstitution at silicon and germanium is expected to favor the backside mechanism, reactionsinvolving free-radical attack at tin are predicted to be less discriminate; indeed, in manycases, the frontside mechanism is calculated to be preferred for reactions involving tin. CCSD(T)/DZP//MP2/DZP calculated energy barriers range from 39.4 kJ mol-1 for the reaction ofsilyl radical with distannane by the frontside mechanism to 104.5 kJ mol-1 for the analogousfrontside reaction involving stannyl radical and disilane. Except for reactions involving attackat the tin atom in methylstannane, we were unable to locate transition states for frontsideattack at correlated levels of theory for reactions involving methyl radical. The mechanisticimplications of these computational data are discussed.
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